For example, in an optical head for a programmable optical recording medium, a collimator lens having a high numerical aperture is normally used in order to improve utilization efficiency of a beam. When a semiconductor laser is used as a light source, an anisotropy in beam irradiation angles is generally corrected using an anamorphic prism.
The semiconductor laser causes a wavelength fluctuation .DELTA..lambda. of 10 to 20 nm due to a change in temperature, output power, and the like, or upon incidence of a return beam from the optical recording medium. A refractive index of the anamorphic prism changes due to the wavelength fluctuation .DELTA..lambda., and as a result, an angle of an exit beam from the anamorphic prism may be changed. However, the conventional anamorphic prism cannot cope with this respect.
In a conventional anamorphic prism which consists of a single prism using an optical glass SF13 and has an enlargement magnification ratio .beta..apprxeq.1.7, if .DELTA..lambda..apprxeq.10 nm, an exit angle change of .DELTA..xi..apprxeq.0.02.degree. occurs. Therefore, when an objective lens having a focal length f=4.5 mm is used, a beam spot is shifted by .DELTA.x=f.multidot..DELTA..xi.=1.5 .mu.m in a lateral direction on the optical recording medium.
The wavelength fluctuation of about .DELTA..lambda..apprxeq.10 nm easily occurs due to various conditions, and cannot be controlled. As a result, it is difficult to perform positional control of the beam spot for a recording or reproducing operation, and a stable and accurate recording or reproducing operation cannot be performed.